Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example. A Fabry-Perot interferometer is based on parallel mirrors, such as quarter wave Bragg reflectors, wherein a Fabry-Perot cavity is formed between the mirrors. The pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the cavity. It is common to use micromechanical technology for producing Fabry-Perot interferometers. Such a solution is described e.g. in patent document F195838.
FIG. 1a illustrates a prior art micromechanical Fabry-Perot interferometer produced on a substrate 130. Mirrors of a micromechanical interferometer usually include several layers 102, 104, 106, 112, 114, 116, wherein materials of adjacent layers have a different refractive index. The micromechanical interferometers used in short wavelength ranges of visible light and near-infrared radiation generally have solid mirror layers, such as a silicon dioxide or silicon nitride layer 104, 114 between silicon layers 102, 106, 112, 116. However, silicon oxide and silicon nitride have relatively high attenuation at long wavelengths, and therefore it is more preferable to use a layer of air between silicon layers in infrared range, especially in the wavelength range over 5 μm, i.e. thermal infrared radiation (TIR).
Movement of the mirror 112, 114, 116 is made possible by removing a sacrificial layer 111 from the optical area A and from a surrounding area around the optical area, whereby a cavity 123 is formed. The sacrificial layer may be e.g. silicon dioxide, which can be removed by etching with hydrofluoric acid (HF), for example. In order to allow the etching substance to reach the sacrificial layer, holes (not shown in FIG. 1a) are provided in the movable mirror. The remaining part of the sacrificial layer serves as a support for the movable mirror. The substrate has optionally been removed from the optical area 125 in order to avoid attenuation and reflection caused by the substrate.
The position of a moveable a mirror is controlled by applying voltage to electrodes, which are included in the mirror structures by making one layer 106, 112 of both mirrors conductive by e.g. doping. There are electrodes 110a and 110b for connecting a voltage to the electrodes. When control voltage is applied between the electrodes of the fixed and movable mirrors this voltage causes a force which moves the movable mirror towards the fixed mirror. If the electrodes cover the whole mirror, the movable mirror will be bent throughout the cavity area. This causes the distance between the movable mirror and the fixed mirror to vary within the optical area A during electrical activation. This is illustrated in FIG. 1b. The non-flatness of the movable mirror within the optical area causes the pass band frequency to vary within the optical area and the bandwidth to become wider. The quality factor of the filter, i.e. finesse, will therefore be reduced. As a result, the finesse of such an interferometer is not sufficiently high for several applications where high finesse is required.
The non-flatness of the movable mirror within the optical area can be avoided by providing the control voltage only outside the optical area of the mirrors. This solution is illustrated in FIG. 1b. The layer 106 is only connected to control voltage at the area outside the optical area, and the voltage applied to the electrode is thus not effective at the optical area. However, there are some disadvantages in this approach as well.
Firstly, due to a smaller electrode area a higher voltage is required between the electrodes in order to achieve sufficient force between the mirrors. It is often difficult to provide high voltages in small-sized sensor circuits, and also energy consumption may increase due to energy losses in a required voltage conversion.
Secondly, even if deflecting voltage is not applied in the optical area it is still necessary to provide electrodes within this area. This is because the optical areas of the mirrors must be connected to a constant voltage potential in order to avoid coupling of static electricity in the optical area, which might cause errors in the mirror position. The movable mirror and the fixed mirror must be in the same electrical potential at the optical area in order to avoid a force between the mirrors in that area. Therefore, a conductive layer of the optical area must be electrically separated from the electrode outside the optical area, and these conductive areas of a mirror must be connected to different voltage potentials. Connecting the conductive areas into different potentials requires providing electrically conductive feed-throughs and leads into several mirror layers. To achieve the electrical feed-throughs and leads a layer, patterning and doping must be applied. As a result the number of micromechanical process phases is increased. This makes the production of the interferometers complicated and makes the production cost too high for cost sensitive applications.
One further disadvantage relates to the shape of the interferometer. The mirrors need to have circular shape because any other lateral shape could cause wrinkling of a tensile-stressed thin-film mirror when it is vertically displaced by the electro-static actuation. On the other hand, circular form of a component is usually not preferable in electronics because the density of the components on a substrate or on an electrical circuit is not optimal.